Pion rejection plot

Create a simple pion rejection plot which selects electrons from a sample of events containing electrons and (charged) pions.

The discrimination between electron/pion should be done with a cut selecting the first fraction (TBD) of the EM shower which will have much more energy in it for electrons. This is a simple "pre-shower" style electron selection.

At this level, we want to keep it very simple. The more sophisticated analysis will come later in the reconstruction benchmarks.